Poster: Exploring the feasibility of glider-based transport, stratification, and ecology measurements on the New England shelf

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Above: The standard glider sensor package included a Sea-Bird CTD, Wet Labs bb2f chlorophyll fluorom-
eter and red/blue optical backscatter sensor, and a custom WHOI PAR (photosynthetically active radia-
tion) sensor. Several vehicles also flew with prototype sensors including a Nortek 1 MHz ADCP, a biolumines-
cence bathyphotometer, a novel fast-response CTD and a low-frequency passive sonar.
Right: The transect connects the shallow (20 m) Martha’s Vineyard Coastal Observatory with Line W, a
deep moored array spanning the Gulf Stream and Deep Western Boundary Current.
Below: Seventeen cross-shelf temperature and salinity sections obtained between October 2006 and
October 2007. Note that each 200 km section corresponds to approximately 7 days of transit time. Most
sections show evidence of the shelfbreak front, a ubiquitous hydrographic feature of the continental
shelfbreak at which cool, fresh shelf water meets the warm and saline water of the continental slope.
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FluorescenceSalinity DensityTemperature
FluorescenceSalinity DensityTemperature
SummerWinter
seasonal changes
A major component of the along-shelf flow and a critical regulator of cross-shelf
transport is the shelfbreak front, visible in both seasons near the 100 m isobath. On
the inner shelf temperature stratification changes dramatically from summer to
winter. Surface heating in early summer results in the formation of a seasonal
pycnocline over the entire shelf and caps the cold, fresh, well-mixed winter water
on the mid-shelf. Fresh surface water on the inner shelf originates in the Gulf of
Maine. The subsurface maximum in chlorophyll fluorescence is elevated in
summer and coincident with the seasonal pycnocline.
velocity and transport
The along- and cross-shelf fluxes of salt, heat, and nutrients in this
region are highly variable and poorly understood. One objective of
this study is to investigate the utility of sustained glider measurements
for estimating these fluxes. Glider-based transport calculations are
complicated by strong tidal velocities and intense, episodic
meteorological forcing, both of which are inadequately resolved by
the slow-moving gliders.
Previous studies have shown that the shelf/slope flow in this region is
predominantly baroclinic. Over the shelf the flow is more barotropic
and hence the reference velocity inferred from the glider is more
important. We have calculated the cross-shelf baroclinic velocity
structure for a section occupied in October 2007. The geostrophic
velocity field contains weak surface intensified flow very near shore, a
broad region of eastward flow over the middle shelf, and a surface
intensified westward jet near the shelf edge. The westward transport
across the section is roughly 0.25 Sv with the majority centered at the
shelfbreak. This is reasonable given that the glider sampled only a
portion of the current.
A more accurate estimate of the velocity structure and resultant
transport will be possible if we can accurately predict and remove
the tidal component of the depth-averaged reference velocity. A
method that uses historical current meter observations to remove the
tides from vessel mounted ADCP records has been used successfully
in this region. We hope to adapt this method to support our analysis
of the glider data.
Above: In order to minimize the small scale cross-shelf density gradients that result from tides and local wind
forcing, the density field (a) is filtered with a 10-km low-pass second-order Butterworth filter. Geostrophic
velocity is calculated from the smoothed density section and referenced to zero at the bottom (b).
Depth-averaged horizontal velocity (c) is estimated from the glider navigation data. A 50-km wide boxcar
filter (roughly 60-hr low-pass) is then applied in order to minimize tidal fluctuations. Finally, the absolute
geostrophic velocity (d, e) is computed by referencing the vertically averaged geostrophic velocity to the
glider-measured slab velocity.
Left: Transport as a function of distance offshore (f) and depth (g) computed from the velocity section in (e)
above.
Johns Hopkins University Applied Physics Laboratory
Gulf of Maine
Gulf Stream
71˚W 70˚W 69˚W 68˚W 67˚W 66˚W
40˚N
41˚N
42˚N
43˚N
44˚N
Georges Bank
Gulf of Maine
Atlantic
Ocean
Boston
Portland
Line W Moored Array
Martha’s Vineyard
Coastal Observatory
44008
summary
The stratification of the continental shelf is critical to a variety of physical and
biological processes and is highly sensitive to both advective and atmospheric
forcing. A thorough understanding of the mechanisms responsible for the
evolution of the shelf stratification on storm-to-seasonal timescales requires
persistent high-resolution measurements over a large spatial footprint.
Previous work, both moored and ship-based, has indicated the need for both
highly-resolved spatial measurements (to describe features on scales of 1-10
km) and a sustained at-sea presence (to capture episodic events and
facilitate robust statistical inferences). Ship-based measurements on the
continental shelf are generally expensive and weather dependent. Moored
observations generally lack spatial resolution.
Autonomous vehicles, when used appropriately relative to their inherent
capabilities, can provide sustained, low-cost, measurements of key physical
and biological variables. However, There are logistical impediments to using
slow, relatively fragile autonomous platforms in a demanding operational
setting such as the New England shelf. These include strong tidal flows,
frequent and intense meteorological events, and vigorous fishing activity. The
goal of this program is to ask “How can these novel assets be most effectively
employed in a coastal observing system?”
Since October 2006 we have used several gliders to occupy a 200 km transect
spanning the New England shelf from the nearshore Martha’s Vineyard
Coastal Observatory (MVCO) to the Line W moored array on the continental
slope. We present here an initial synthesis of nearly 20 independent
realizations of this section, consider seasonal variations in stratification, and
evaluate our ability to compute along-shelf baroclinic transport.
Left: Seasonal changes in stratification are evident in sections from (top) July 2007
and (bottom) January 2007.
Below: A pronounced annual cycle is evident in air and sea temperature from
NDBC buoy 44008 located near the midpoint of the glider section.
Jan06 Apr06 Jul06 Oct06 Jan07 Apr07 Jul07 Oct07 Jan08
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Exploring the feasibility of glider-based transport, stratification, and
ecology measurements on the New England shelf
John M. Lund, Paula Sue Fratantoni, Benjamin A. Hodges, and David M. Fratantoni
Autonomous Systems Laboratory, Physical Oceanography Department
Woods Hole Oceanographic Institution, Woods Hole, MA 02543
dfratantoni@whoi.edu
Acknowledgements: This work was supported by the Woods Hole Oceanographic Institution’s Coastal Ocean Institute and WHOI’s Access-to-the-Sea Program. We thank the captains of the research vessels Mytilus and Tioga
for assistance with deployment and recovery operations. Vehicles used in this program were acquired with support from the Office of Naval Research. aSLaSLAutonomous Systems Laboratory
Woods Hole Oceanographic Institution